1. Field of the Invention
This invention relates to a highly selective method of producing formaldehyde, HCHO, by the partial oxidation of methane using a special type of catalyst and to a unique, highly selective formaldehyde-forming catalyst.
2. Description of the Previously Published Art
The major commercial process to make formaldehyde is from methanol. This involves the steam reforming of methane with high grade heat to convert the methane into syn gas. The syn gas is reacted to form methanol while giving off low grade heat. Then the methanol is oxidized to formaldehyde while giving off additional low grade heat. This synthesis procedure requires multiple steps and it involves poor energy usage.
In the past a small fraction of formaldehyde was made by the partial oxidation of lower petroleum hydrocarbons which involved the partial oxidation of the hydrocarbon gas with air or oxygen under pressure, followed by rapid cooling, condensation, and absorption of the products in water to give a crude solution, which must then be refined to separate formaldehyde from the other reaction products, such as methanol, acetaldehyde, propyl alcohol, propionaldehyde, and organic acids. Formaldehyde is isolated as a dilute solution, which must be concentrated to market strength. Propane and butane are the basic hydrocarbon raw materials for formaldehyde. Products manufactured by oxidation of propane and butane include formaldehyde, acetaldehyde, acetone, propionaldehyde, methanol, n-propyl alcohol, isopropyl alcohol, and butyl alcohols
The German Offenlegungsschrift No. 2,404,738 to Bayer discloses oxidizing methane to formaldehyde by using many different types of metal oxides which may be placed on many different types of supports. The metal oxides are in Groups V, VI and/or VII of the Periodic Table. Among those listed are oxides of vanadium, niobium, tantalum, chromium, uranium, molybdenum, tungsten, manganese, technetium and rhenium, mixtures of these oxides with each other, and mixtures of these oxides with other oxides such as silica, alumina, iron oxide, calcium oxide, magnesia, sodium oxide or potassium oxide. In the first example using methane, the catalyst is 10 wt % Mo as MoO.sub.3 on silica.
This reference is not helpful in finding an optimum methane conversion catalyst. It provides no attention as to the criticality of the support. It groups silica with alumina and other metal oxides yet, as will be shown in the following examples, alumina when used as a support is not as effective as the catalyst of the present invention.
Furthermore the Bayer reference provides no attention to the criticality of having a low sodium concentration. As will be shown in the following Comparison Example 1 and reported in Table 1, even when the Bayer molybdenum oxide-silica combination is used, as in Example 1 of the German patent application, the selectivity to formaldehyde is lower than that obtained with the present invention. This difference in result is believed due to the significant levels of sodium which can be present in silica. Sodium is present in most forms of silica that are synthesized in aqueous media because the conventional starting materials contain sodium. Offenlegungschrift 2404738 goes so far as to specify that up to 5 wt % Na.sub.2 O may be present in the working catalyst. As will be shown in Examples 7-10, sodium in these concentrations has a profoundly deleterious effect on selectivity to formaldehyde in the partial oxidation of methane.
U.S. Pat. No. 3,996,294, to Imre et al and assigned to Bayer discloses the use of a silica catalyst which does not contain any molybdenum to produce formaldehyde from methane. As will be shown in the examples, a catalyst consisting purely of silica, such as Cabosil (a product of Cabot Corporation), exhibits a lower selectivity to formaldehyde than is obtained with the catalyst of the present invention. Some of the examples in the Imre et al patent employ catalysts made substantially of silica along with small amounts of other metal oxides. There is no example given of using molybdenum oxide, although, as in the companion German Offenlegungsschrift, there is a broad list of metal oxides which can be used, including oxides of aluminum, iron, vanadium, molybdenum, tungsten, calcium, magnesium, sodium or potassium with a specific reference that up to 5% of Na.sub.2 O can be used.
Japanese Patent Publication No. 58-92629 discloses a SiO.sub.2 -MoO.sub.3 catalyst, using N.sub.2 O or oxygen as oxidants. They report negligible selectivity to methanol or formaldehyde in the case of O.sub.2 oxidation. In the publication of Liu et al (J. Chem. Soc. Chem. Commun. 1982, 78 and J. Am. Chem. Soc. 1984, 106, 4117) a molybdena-silica catalyst is described, which also uses nitrous oxide for the partial oxidation of methane to formaldehyde and methanol. The use of N.sub.2 O as an oxidant is prohibitively expensive. Also, the silica in the catalyst is a fumed silica which is an expensive form of silica.
British Pat. No. 1398385 discloses a methane partial oxidation catalyst consisting of MoO.sub.3 and CuO in the absence of carrier or binder. The inventors state, however, that the presence of at least 2 vol. % of a higher hydrocarbon must be present for the process to be effective, and their principal oxygenated product is methanol. Formaldehyde selectivity reaches a maximum of 15.5%.
British Pat. No. 1244001 discloses several catalysts for the partial oxidation of hydrocarbons. All of these catalysts contain MoO.sub.3. The inventors claim that the principal oxygenated product is methanol, and the maximum selectivity to formaldehyde is 8%.